Radioactivity detector



March 13, 1951 J. D. LAHMEYER ET AL RADIOACTIVITY DETECTOR Filed Feb. 8,1949 FIG. 2

RECORDER AMPLlFlER 72% 17. Lafimeyer BY 6%, f lin v ATTORNEYS PatentedMar. 13, 1951 RADIOACTIVITY DETECTOR John D. Lahmeyer, Tulsa, Okla., andJohn T. Callahan, 'La Feria, Tex., assignors to Industrial NucleonicDevices, Tulsa, Okla., a corporation of Oklahoma Application February 8,1949, Serial No. 75,248

4 Claims. 1

This invention relates to radioactive detectors, and particularly toionization chambers therefor. The invention is particularly directed tolow voltage ionization chambers, but in certain aspects is of moregeneral application.

Nuclear counters for detecting radioactivity may be divided into (1) lowvoltage ionization chambers, (2) proportional counters operating athigher voltages and ('3) Gieger counters operating at still highervoltages. The present invention is particularly designed to provide animproved ionization chamber of the first-mentioned category, althoughcertain features are of broader application. Difiicultles have beenencountered with ionization chambers due to their relatively lowsensitivity, spurious responses, and proneness to drift. It is a primaryobject of the present invention to provide an ionization chamber whichis markedly improved in these respects.

In accordance with the present invention an ionization chamber isprovided having a thin diaphragm Window and an arrangement of electrodeswhich give marked directivity to the device, particularly for beta andsoft gamma rays. The chamber is formed almost entirely of a singlemetal, and the construction so designed as to reduce to a minimum theneed for different types of metals in the interior thereof. Hence thedanger of spurious responses due to electric potentials established bycontact between different metals which might give rise to thermocoupleor other electrical effects is minimizzd. In accordance with anotherfeature of the invention, the first tube of the electrometer circuit,together with its input resistance, is mounted inside the ionizationchamber in such a manner as to reduce the capacitance shunting the inputcircuit and also the leakage paths shunting the input resistance. Inthis manner the sensitivity of the device is markedly increased, thetime constant of the input circuit is reduced without requiring areduction in the input resistance, and the likelihood of spuriousresponses and drift is greatly reduced.

The invention will be more fully understood by reference to thefollowing detailed description of a specific embodiment thereof, takenin conjunction with the accompanying drawings in which:

Fig. 1 is an elevation of portable apparatus containing an ionizationchamber;

Fig. 2 is a vertical cross section of the ionization chamber itself,taken along the line 2-2 of Fig. 1;

Fig. 3 is a cross section of the ionization chamber taken along the line3- 3 of Fig. 1;

Fig. 4 is a detail of a lead-through seal; and

Fig. 5 is a circuit diagram of the ionization chamber in an appropriateamplifier and recorder circuit.

Referring to Fig, 1, an ionization chamber [0 is shown mounted in thelower portion of a cylindrical container H provided with a handle I2.

' The container ll maybe of any suitable durable material, preferablynonconductive, and is provided with an inner cylindrical shield 13 whichmay be of sheet'steel. The ionization chamber is mounted in the lowerend of the container II with a thin diaphragm Window M at the bottom endthereof. The container is shown resting on the ground I 5 so as to pickup'radiations emanat ing therefrom.

The ionization chamber comprises a cylindrical wall 2! of an appropriatemetal. It is preferred to employ aluminum since it is readily availablein a very pure state and has relatively low alpha emission. Thisdecreases spurious responses and background noise. If desired, othersuitable metals could be employed. The cylindrical wall 2| is providedwith end plates 22 and 23, advantageously of the same metal. Circulargrooves 24 and 25 are provided in the end plates to receive the cylinder2i. Advantageously sealing rings of rubber or other material 26 and 21are inserted in the circular grooves to provide an air-tight seal. Theend plates are secured in position by means of bolts 28 arranged aroundthe periphery thereof.

The lower end plate 22 is provided with a thin diaphragm window Madvantageously formed integrally in the end plate. This may beaccomplished by starting with a metal disc of desired thickness andmachining away the inner portion thereof to form the window. A diaphragmthickness of about 0.002 inch has been employed with success, and thisthickness of aluminum permits beta and soft gamma rays to pass freelyinto the chamber. Naturally the more penetrating hard gamma and-cosmicrays can also enter the chamber.

'- connected to the lead-through conductors and 3 serve as the tubesupport. The flexibility of the leads aid materially in preventingdamage to the tube due to shock, etc. If desired, a recess 36 (Fig. 3)may be provided in the end plate and soft damping material 3? placedtherein in contact with the adjacent end of the tube 3| so as to preventexcessive vibration.

The tube 3| is so mounted that the grid lead 32 extends away from theend plate 23 and toward the window M to form a slender centralcollecting electrode for the ionization chamber. The tip 38 is locatednear the diaphragm window M at a suitable distance therefrom.Advantageously the wire 32 is coaxial with wall 2|. The input resistance33 is also located within the ionization chamber with one end 4|connected to the grid lead 32, and the other end 42 connected to aleadthrough conductor. Preferably the junction of wires 4| and 32 isnear the tip 38 of the collector electrode. The resistance 39 is of veryhigh value and may be encased in a sealed glass tube 43 to preserve thevalue of the resistance.

The ionization chamber is filled with a suitable gas by means of valves44 and 45. Low gas pressures are ordinarily employed, of the order of afew pounds above atmospheric. Any suitable gas may be employed, such asargon, nitrogen, hydrogen, etc. Under some circumstances air alone maybe employed. The window diaphragm I4 is shown as slightly dished, due tothe pressure in the chamber and slight stretching of the metal duringmachining. It may, of course, be fiat if desired.

It will be observed that the interior of the chamber is bounded bysurfaces which are almost entirely of one metal, advantageouslyaluminum. Due to the type of construction employed, the use of othermetals for soldering, etc. is avoided. Also, insulating surfaces withinthe chamber are kept to a minimum so as to avoid the collection ofstatic charges thereon which might discharge periodically to givespurious responses. The sealing rings 26 and 21 are embedded in the ringgrooves so as to minimize any collection of static charges thereon.Advantageously the exposed surfaces of valves 44 and 45 extending withinthe chamber are of the same metal as the rest of the chamber.

The provision of proper seals for the leadthrough conductors 35 isimportant. It is desirable to avoid the use of different kinds of metalin the lead-through connections in order to avoid electrical actionbetween the metals which might give rise to spurious responses. Thelead-through seal shown in Fig. 4 is particularly advantageous. Asshown, the conductor 35 is sealed in a glass bead Also sealed to theglass bead 5| is a metal ring 52. The end plate 23 is drilled to receivethe glass bead 5|, and is provided with an annular threaded socket 53surrounding the bead. A rubber O ring 54 is seated in the socket 53 andthe ring 52 held thereagainst by the annular ring-screw 55. Bytightening screw 55 a gastight seal can be readily obtained. Ring-screw55 is on the inside of the chamber and is advantageously of the samematerial as the end face 23. Hence, when the lead-through connection hasbeen secured in place, the metal ring-screw 55 is exposed to theinterior of the chamber. This avoids the necessity for using other typesof material for soldering, brazing, etc. Furthermore, the O ring 54 iscovered by the metal ringscrew and hence presents no surface which mightcontaminate the gas in the chamber or collect static charges.

A metal can 56 may be placed around the outer terminals of thelead-through connection and may contain additional amplifying stages.Inasmuch as the output of an ionization chamber is very small, shieldingis highly desirable and this is provided by can 55. After the initialamplification, the output may be fed to subsequent amplifier stagesthrough cable 51.

Referring now to Fig. 5, the ionization chamber is showndiagrammatically with the outer wall indicated as 2| and the centralcollecting electrode indicated as 38. The electronic tube 3| is shownseparate from the ionization chamber for convenience, but will beunderstood to be actually inside the chamber as shown in Fig. 2. Asecond thermionic tube 6|, preferably identical with tube 3|, isprovided to balance the no-signal current of tube 3| so that the outputsignal represents essentially only the A.-C. signal, The heater filamentleads 32 of tube 3| are connected in series with. the heater filament oftube 6| and energized by suitable battery 82. Grid resistor 39 isconnected from grid to filament through a 0 battery 63 which providessuitable negative bias. Load resistors 64 and 65, preferably of equalvalue, are connected to the anodes of tubes 3| and BI, and to thefilaments through the B battery 66. Resistors 54 and 65 are shunted bylarge electrolytic capacitors 67 and 68. A suitable potential is appliedbetween the chamber wall 2| and collector electrode 38 of the ionizationchamber by means of batteries 63, 66 and 69, through grid resistor 39and resistor II. An adjustable grid bias potential is applied to tube 6|through battery 12 and battery 13 shunted by potentiometer 14. Abalanced output signal is supplied to amplifier 12 through resistors 19and 86. The mid-point 15 between plate resistors 64 and 35 is groundedand connected to amplifier 12 through an RF choke 16. The output ofamplifier i2 is fed to a suitable recorder 11.

Potentiometer i4 is initially adjusted so that the anode currents fromtubes 3| and 6| flowing through respective resistors 64 and 65 provideequal and opposite voltages so that the resultant signal applied toamplifier 12 is substantially zero. When the ionization chamber issubjected to radiation, the signal is amplified b tube 3| and suppliedto amplifier 12. The general arrangement of the circuits in Fig. 5 arewell known and need not be described further.

Resistor 39 is of very high value, a value 2 10 ohms having beenemployed successfully.

The capacitance of the ionization chamber and associated wiring resultsin a distributed capacitance which is efiectively in shunt with resistor39 as shown by the dotted condenser 18. It is desirable to keep thecapacitance of the chamber and associated wiring as low as possible inorder to obtain maximum sensitivity in the device. Generally speaking,the sensitivity varies in inverse proportion to the capacitance. Also,the time constant RC of capacitance T8 and resistor 39 is important. If,for example, R were 2 10 ohms and C were 59 mmf., a time constant of 109seconds would result. Such a capacitance would ordinarily be obtainedwhen the tube 3| and resistor 39 are placed outside the ionizationchamber and the collector electrode is led through the end face 23. Itis difficult to obtain lead-through capacitances much less than mmf. ()nthe other hand, when, in accordance with the invention, the tube 3| isplaced in to side the chamber as shown in Fig. 2, the capacitance "fromthe collecting electrode '38 to the chamber wall 'is exceedingly low,'By placing res'istor 39 inside the chamber also, "the net capacitancein shunt with the resistor is greatly reduced. Values of 4 mmf. or lessare readily obtained. With such a low capacitance, time constants ofeight seconds or less are easily obtained, thereby greatly increasingthe speed of taking readings. At the same time the sensitivity of thechamber is increased. It will therefore be un derstood that thereduction in capacity obtained in accordance with the present invenLiongreatly increases the sensitivity and usefulness of the ionizationchamber.

A further advantage in the present arrangement is that leakage currentswhich would decrease the sensitivit or the device are largely avoided.Consider again the casein which the tube 31 and resistor 39 are placedoutside the ionization chamber, thereby requiring the collectorelectrode to be led through the chamber wall to the grid of the tube.Representative voltages which have been employed with success are +45volts to ground for the chamber wall and 25 volts to ground for the gridof tube 3|. This gives a total of approximately '70 volts from thecollector electrode to the chamber wall. The currents resulting fromionization in a chamber of the type described are very minut and theinput resistor 39 is very large. Hence, even with a low voltage such as'70 volts the magnitude of leakage current which could flow from thecollector lead through the chamber wall might easily be of the sameorder of magnitude as the ionization current. This leakage current wouldby-pass the resistor 39 and hence reduce the sensitivity of the device.When, in accordance with the invention, tube 3| and resistor 39 areplaced inside the chamber, this leakage path is eliminated. Consideredfrom another viewpoint, in the arrangement shown in Fig. 2 any currentwhich could flow between the grid lead-through conductor and the endwall 23 must first pass through resistor 39 and hence yield a signalvoltage. The profecting glass tube 43 may be carefully cleaned beforeinserting it into the ionization chamber so as to reduce any leakagetherealong to a negligible value, and the ionization chamber wallprotects the resistor from contamination during use.

As mentioned before, the ionization chamber of the invention has markeddirectivity, particularly for beta and soft gamma rays. This resultsfrom the employment of the thin window M at one end of the cylindricalwall 2i forming the outer electrode, in conjunction with the tip 38 0fthe inner collecting electrode extending toward and fairly close to thewindow. The combination of this arrangement with a direct connectionbetween the inner electrode and the grid of tube 3| and resistor 39results in an ionization chamber of excellent sensitivity to beta andgamma rays, with relatively low spurious response. Furthermore, byplacing the tube and resistor inside the chamber so as to reducedistributed capacitance and leakage, the chamber is made lesssusceptible to drift, inasmuch as changes in the values of the circuitconstants are minimized.

It is sometimes found that when the equipment is operated underdifferent temperature conditions, tube 6! can advantageously be placedinside the ionization chamber along with tube 3|. Tube 6| will then bein the same environment as tube 3| and the change in balance of 6 theen-edit of Fig. 5 due to temperature and atmospheric conditions areminimized.

"The invention has been described hereinbefore in connection with aspecific embodiment thereof. It will be obvious to those skilled in theart that many variations are possible within the spirit of theinvention, the "scope of which is defined in the accompanying claims.

"We claim: r v

1. A radioactivity detector comprising a cylin 'dri'cal wall and endplates therefor cooperating t'o define a closed ionization chamber, saidcylin"- d'fic'al wall forming one electrode of the chamber, a thindiaphragm window in one of said end plates, an electronic tube having atleast an anode, cathode and control grid mounted inside said chamber andcentrally located with the other of said end plates, a slender innerelectrode extend,- ing from said grid toward said diaphragm window'wiifi'i theend thereof substantially coaxial with the cylinder, and aninput resistance mounted in said chamber and connected to the grid ofsaid tube, whereby a directional radioactivity detector of highsensitivity may be obtained.

2. A radioactivity detector comprising a cylindrical Wall and end platestherefor of a selected metal cooperating to define a closed ionizationchamber, said cylindrical wall forming one electrode of the chamber, athin diaphragm window in one of said end plates, a plurality ofleadthrough conductors sealed in the other end plate, an electronic tubehaving at least an anode, cathode and control grid mounted inside saidchamber centrally of said other end plate, a slender inner electrodeextending substantially coaxially from said grid toward said diaphragmwindow, an input resistance mounted in said chamber and connected tosaid inner electrode near the end thereof toward the window, andelectrical connections from said lead-through conductors to said anode,cathode and resistance respectively, whereby a directional radioactivitydetector of high sensitivity and low capacitance and leakage may beobtained.

3. A radioactivity detector comprising a cylindrical wall and end platestherefor of a selected metal cooperating to define a closed ionizationchamber, said cylindrical wall forming one electrode of the chamber,athin diaphragm window in one of said end plates, a plurality ofleadthrough conductors sealed in the other end plate, an electronic tubecomprising an envelope and at least anode, cathode and control grid,said tube being mounted inside the chamber centrally of said other endplate, a slender inner electrode extending substantially coaxially fromsaid grid toward said diaphragm window, an input resistance mounted insaid chamber and connected to said inner electrode near the end thereoftoward the window, electrical connections from said lead-throughconductors to said anode, cathode and resistance respectively, anionizable gas at low gauge pressure in said chamber, and means forapplying a relatively low voltage between said electrodes to form anionization chamber whose response varies with the ionizing ability ofincident radiation,

4. A radioactivity detector comprising a cylin drical wall and endplates therefor of a selected metal cooperating to define a closedionization chamber, said cylindrical wall forming one electrode of thechamber, a thin diaphragm window in one of sa d end plates and integraltherewith, a plurality of lead-through conductors, each con- 7 ductorhaving an insulating bead sealed therearound and an annular disk sealedaround the bead, threaded annular sockets in the other end platereceiving said conductors from the interior surface thereof, a resilientsealing ring between the annular disk of each conductor and the bottomof the respective socket, a ring-screw of said selected metal clampingeach of said conductors in the respective socket, an electronicamplifier tube comprising an envelope and at least anode, cathode andcontrol grid, said tube being mounted inside the chamber centrally ofsaid other end plate, a slender inner electrode extendingsubstantiallycoaxially from said grid toward said diaphragm window, an input resistance mounted in said chamber and connected to said inner electrode nearthe end thereof toward the window, electrical connections from saidlead-through conductors to said anode, cathode and resistancerespectively, an ionizable gas at low gauge pressure in said chamber,and means REFERENCES CITED The following references are of record in thefile of this patent:

UNITED STATES PATENTS Name Date Kallmann et al. July 7, 1942 OTHERREFERENCES Copp et al.: Review of Scientific Instruments, July 1943, pp.205-206.

Brubaker and Pollard: Review of Scientific In. struments, July 1937, pp.254-256 Number

